Projection lens for a projector

ABSTRACT

A projection lens for a projector according to the present invention comprises, in order from the screen side, the first lens unit including at least one positive lens and negative lens, the positive second lens including at least one negative lens and positive lens, and the negative third lens unit with a concave surface having a large curvature directed at the screen side, the first lens unit merely comprising spherical lens elements, at least one of the negative lenses in the first lens unit having a concave surface of larger curvature directed at an adjacent convex surface at the object side (CRT side), the second lens unit or the third lens unit having at least one or more aspherical surfaces. Thereby, an image quality of high definition as in a high quality television is realized without employing an aspherical surface is a portion where error in fabrication affects greatly aberrations due to the aperture, the affection tending to occur when an aspherical lens is used in the first lens unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a projection lens for a projector, and more particularly to a bright projection lens of high definition which is suitable for a projection lens for a television projector of high quality.

2. Description of the Prior Art

A television projector is designed so that a picture plane of a cathode ray tube is enlarged and projected on a screen by a projection lens as is known. A demand of such a projector has been recently increased not only for business use but also for home use and the wide-use thereof has been expected. With this trend, projector devices of more compact size and lower cost have been greatly desired. A projector lens also becomes necessary to have a large aperture ratio, approximately 1:1.2, and an extremely wide angle.

On the other hand, a sign now appears in which a so-called high quality television having a resolution equal to that of a 35 mm movie has been put to practical use mainly for business use, and expected to come into wide use for home use. Under these circumstances, also in the television projector, the demand toward the high quality of image is particularly high, and the image quality of high definition as seen in the high quality television has been desired also for the television projector.

Projector lenses in an attempt of obtaining an image quality of high definition in response to such a demand as described are disclosed as examples thereof in unexamined published Japanese Patent Application Nos. 106427/1987, 85212/1987 and so forth. In these lenses, as can be seen in their examples, an aspherical lens in used for the first lens unit whereby the spherical aberration, the coma and the like due to the aperture are corrected.

Actually, however, the technique for preparing aspherical lenses has not yet been established, and it is very difficult to prepare them even at present. Further, even in the case where an aspherical lens formed from plastic which is relatively simply worked is used, the environmental characteristics are inferior to that of a glass lens. Therefore, particularly in the case where an aspherical lens is used for a portion as the first lens unit in which an error in fabrication greatly influences on aberrations due to the aperture, the spherical aberration is greatly affected, and a higher order flare occurs to lower contrast. As a result, the image quality is deteriorated to make it difficult to realize a projector of high definition.

In view of the foregoing, it is contemplated that a spherical lens is used for the first lens unit in order to solve the aforementioned problem. However, it has been heretofore essential to use aspherical surfaces for the first lens unit in order to realize the image quality of high definition in terms of design. Under these circumstances, a projection lens for a projector capable of correcting various aberrations in a satisfactory manner while using a spherical lens for the first lens unit has been demanded.

SUMMARY OF THE INVENTION

This invention is intended to provide a projection lens for a projector in which various aberrations can be well corrected while using a spherical lens as the first lens unit, and in which a high definition image quality as in a high quality television is obtained with relatively less number of lenses.

A projection lens for a projector according to the present invention comprises, in order from the screen side, the first lens unit including at least one positive lens and a negative lens, the positive second lens unit including at least one negative lens and a positive lens and the negative third lens unit with a concave surface having a large curvature directed at the screen side, the first lens unit merely comprising spherical lens elements, at least one of the negative lenses in the first lens unit having a concave surface having larger curvature directed at an adjacent convex surface at the object side (CRT side), the second lens unit or the third lens unit having at least one or more aspherical surfaces.

In the projector lens according to the present invention, aberrations due to the aperture are well corrected by the first lens unit including, in order from the screen side, at least one positive lens and a negative lens, chromatic aberrations including magnification is well corrected by the negative lens in the first lens unit, and at least one of the negative lenses has a concave surface having larger curvature directed at an adjacent convex surface at the opposite side of the screen to make petzval's sum of the entire system small. The second lens unit is a positive lens unit including at least one negative lens and a positive lens, the lens unit bearing a considerable portion of refracting powder of the entire system. At least one negative lens in the second lens unit makes it possible to correct the chromatic aberration in a satisfactory manner. In the third lens unit, an image surface is well corrected by a negative lens unit having a concave surface having a large curvature directed at the screen side.

Further, the first unit is composed of spherical lens elements, thereby rendering the designed performance reproducible in the practical use, and at least one aspherical surface is used for the second lens unit or the third lens unit which is close to the object, small in width of ray bundle and less sensitive to error to thereby effectively correct various aberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the lens structure of Examples 1 to 3 and 5 of a projection lens according to the present invention;

FIG. 2 is a sectional view showing the lens structure of Example 4;

FIGS. 3 to 7 show aberrations of Examples 1 to 5, respectively;

FIG. 8 is a sectional view showing the lens structure of Example 6;

FIG. 9 is a sectional view showing the lens structure of Example 7;

FIG. 10 is a sectional view of the lens structure of Example 8; and

FIGS. 11 to 13 show aberrations of Examples 6 to 8, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of a projection lens for a projector according to the present invention will be shown hereinafter. In Examples 1 to 5 of the projector lens, the first lens unit in which all lenses are spherical elements has an arrangement including, in order from the screen side, a positive single lens, a positive meniscus single lens and a negative single lens, and the second lens unit includes a positive lens which bears a considerable refracting power of the entire system, a negative lens and a positive lens. A third lens comprises a negative lens with a foncave surface having a large curvature directed at the screen side. The second lens unit or the third lens unit has at least one aspherical surface. This projection lens comprises seven elements in total.

The projector lens according to these Examples is preferable fulfilled, as auxiliary conditions, with the following conditions:

(1) ν₁₋ <40, ν₂₋ <40

where

ν₁₋ : Abbe's number of a negative lens in the first lens unit

ν₂₋ : Abbe's number of a negative lens in the second lens unit

(2) 0.7 <f/f_(2max) <1.5

f: Focal length of the entire system

f_(2max) : Focal length of a positive lens having the largest refracting power in the second lens unit

(3) A glass lens is used for the positive lens having the largest refracting power in the second lens unit.

(4) The second lens unit includes a plastic positive lens and a plastic negative lens.

The condition (1) is the condition relating to Abbe's number of the negative lens including in the first lens unit and the second lens unit. In order that the chromatic aberrations including the magnification may be well corrected, it is effective and desirable to fulfill this condition.

The condition (2) is the condition relating to the relationship between the focal length of the entire system and the focal length of the positive lens having the larqest refracting power in the second lens unit. When the value exceeds the upper limit, occurrence of the introvert coma increases, and when the value exceeds the lower limit, occurrence of the extrovert coma increases. Particularly in the case where the second lens unit is composed of spherical lens elements, it is desirable that this condition is fulfilled.

The condition (3) is the condition relating to lens material of the positive lens having the largest refracting power in the second lens unit. This lens can be formed from a glass lens to thereby improve the characteristic concerning temperature change of the lens in which aberrations are greatly sensitive to error.

The condition (4) is the condition relating to the positive lens and the negative lens in the second lens unit. In order to reduce cost in the second lens unit, plastic lenses are used in the case where satisfactorily maintain the temperature characteristic, a pair of a plastic positive lens and a plastic negative lens are preferably used.

In Tables of Examples given below, ri represents the radius of curvature of apex of the i-th lens surface from the screen side; di, the i-th space between lens surfaces from the screen side; ni, the refractive index of wave-length 546.07 nm (e line) (for examples 1 to 5), and wavelength 587.56 nm (d line) (for examples 6 to 8), of the i-th lens material from the screen side; and ν_(i), the Abbe number of the material. In the rectangular coordinates in which the apex of the surface serves as an origin and the direction of optic axis is represented by the axis X, the aspherical shape is represented by ##EQU1## where C represents the curvature of the apex, K the conical constant, Ai the aspherical coefficient, and Pi(Pi>2.0) the exponent of the aspherical surface.

In the Tables, values of a liquid layer (Liq) and a face plate (FP) are also indicated.

EXAMPLE 1

f=136.06 Aperture ratio 1:1.2 Magnification -0.125268 Half field angle 21.75° f/f_(2max) =1.3

    ______________________________________                                                                  ri         di   ni     νi                          ______________________________________                                                   1              188.804    13.00                                                                               1.72391                                                                               43.4                                     2              -16170.780 2.20                                                 3              80.810     15.28                                                                               1.69661                                                                               53.0                           G.sub.1   4              180.905    7.01                                                 5              1321.456   4.38 1.73429                                                                               28.2                                     6              73.787     49.91                                                7              95.025     27.00                                                                               1.60548                                                                               60.5                                     8              -170.859   7.86                                                 9              -158.489   3.50 1.58975                                                                               30                             G.sub.2   10             -519.291   35.97                                                11             370.742    5.00 1.49410                                                                               57                                       12             -326.152   30.46                                                13             -60.847    3.50 1.58975                                                                               30                                       14    Liq      -90.000    15.99                                                                               1.41000                               G.sub.3   15    EP       ∞    7.00 1.54212                                         16             ∞                                               ______________________________________                                         Aspherical coefficient                                                                               Exponent                                                 ______________________________________                                         11th surface                                                                   K = 0.0                                                                        A1 = -4.79682 × 10.sup.-7                                                                      P1 =  4.000                                              A2 = -1.00774 × 10.sup.-10                                                                     P2 = 6.000                                               A3 = -2.63511 × 10.sup.-15                                                                     P3 = 8.000                                               A4 = -8.40265 × 10.sup.-18                                                                     P4 = 10.000                                              12th surface                                                                   K = 0.0                                                                        A1 = -6.26348 × 10.sup.-8                                                                      P1 = 4.000                                               A2 = 1.21598 × 10.sup.-11                                                                      P2 = 6.000                                               A3 = -3.59835 × 10.sup.-15                                                                     P3 = 8.000                                               A4 = 8.93349 × 10.sup.-18                                                                      P4 = 10.000                                              13th surface                                                                   K = -6.63848 × 10.sup.-1                                                 A1 = 2.91461 × 19.sup.-7                                                                       P1 = 4.000                                               A2 = -5.70343 × 10.sup.-11                                                                     P2 = 6.000                                               A3 = 6.52599 × 10.sup.-14                                                                      P3 = 8.000                                               A4 = -1.38890 × 10.sup.-17                                                                     P4 = 10.000                                              ______________________________________                                    

EXAMPLE 2

f=136.07 Aperture ratio 1:1.2 Magnification -0.125268 Half field angle 21.92° f/f_(2max) =1.26

    ______________________________________                                                                  ri       di    ni     νi                           ______________________________________                                                   1              197.713  13.00 1.72391                                                                               43.7                                      2              33089.800                                                                               0.98                                                   3              82.680   15.28 1.69661                                                                               53.2                            G.sub.1   4              195.221  7.01                                                   5              1493.969 4.38  1.73429                                                                               28.5                                      6              75.281   53.01                                                  7              96.073   27.00 1.60548                                                                               60.7                                      8              -181.754 8.25                                                   9              -164.115 3.50  1.58975                                                                               30                              G.sub.2   10             -430.011 34.95                                                  11             407.333  5.00  1.49410                                                                               57                                        12             -349.353 33.82                                                  13             -74.735  3.50  1.58975                                                                               30                                        14    Liq      ∞  15.99 1.41000                                G.sub.3   15    FP       ∞  7.00  1.54212                                          16             ∞                                               ______________________________________                                         Aspherical coefficient                                                                               Exponent                                                 ______________________________________                                         11th surface                                                                   K = 0.0                                                                        A1 =  -4.18903 × 10.sup.-7                                                                     P1 = 4.000                                               A2 = -7.38150 × 10.sup.-11                                                                     P2 = 6.000                                               A3 = -1.21502 × 10.sup.-14                                                                     P3 = 8.000                                               A4 = -9.43177 × 10.sup.-18                                                                     P4 = 10.000                                              12th surface                                                                   K = 0.0                                                                        A1 = 3.29803 × 10.sup.-8                                                                       P1 = 4.000                                               A2 = 1.11107 × 10.sup.-11                                                                      P2 = 6.000                                               A3 = 1.28598 × 10.sup.-15                                                                      P3 = 8.000                                               A4 = 2.31801 × 10.sup.-18                                                                      P4 = 10.000                                              13th surface                                                                   K = -5.42427 × 10.sup.-1                                                 A1 = 5.05489 × 10.sup.-8                                                                       P1 = 4.000                                               A2 = 1.02948 × 10.sup.-11                                                                      P3 = 6.000                                               A3 = 6.56394 × 10.sup.-14                                                                      P3 = 8.000                                               A4 = -1.46357 × 10.sup.-17                                                                     P4 = 10.000                                              ______________________________________                                    

EXAMPLE 3

f=136.16 Aperture ratio 1:1.2 Magnification -0.125268 Half field angle 21.70° f/f_(2max) =1.26

    ______________________________________                                                                  ri        di    ni    νi                           ______________________________________                                                   1              180.332   15.00 1.72391                                                                              43.4                                      2              879.973   0.30                                                  3              89.312    15.28 1.69661                                                                              53.0                            G.sub.1   4              284.319   7.01                                                  5              -5553.780 4.38  1.73429                                                                              28.2                                      6              78.754    48.87                                                 7              95.133    27.63 1.60548                                                                              60.5                                      8              -187.531  7.13                                                  9              -177.907  3.50  1.62409                                                                              36.0                            G.sub.2   10             -369.324  24.75                                                 11             595.202   30.00 1.49410                                                                              57                                        12             -243.032  28.48                                                 13             -70.293   3.50  1.58975                                                                              30                                        14    Liq      ∞   8.41                                        G.sub.3   15    FP       ∞   16.62 1.54212                                         16             ∞                                               ______________________________________                                         Aspherical coefficient                                                                               Exponent                                                 ______________________________________                                         11th surface                                                                   K = 0.0                                                                        A1 = -3.59649 × 10.sup.-7                                                                      P1 = 4.000                                               A2 = -5.16003 × 10.sup.-11                                                                     P2 = 6.000                                               A3 = -6.27843 × 10.sup.-15                                                                     P3 = 8.000                                               A4 = -4.04854 × 10.sup.-18                                                                     P4 = 10.000                                              13th surface                                                                   K = -2.62660 × 10.sup.-1                                                 A1 = -1.82120 × 10.sup.-7                                                                      P1 = 4.000                                               A2 = 3.40875 × 10.sup.-11                                                                      P2 = 6.000                                               A3 = 2.79964 × 10.sup.-14                                                                      P3 = 8.000                                               A4 = -1.46647 × 10.sup.-17                                                                     P4 = 10.000                                              ______________________________________                                    

EXAMPLE 4

f=136.18 Aperture ratio 1:1.2 Magnification -0.125268 Half field angle 23.18° f/f_(2max) =1.19

    ______________________________________                                                                  ri        di    ni    νi                           ______________________________________                                                   1              182.866   15.00 1.72391                                                                              43.4                                      2              -4414.347 0.41                                                  3              96.423    15.28 1.69661                                                                              53.0                            G.sub.1   4              289.896   7.01                                                  5              -3000.000 4.38  1.73429                                                                              28.2                                      6              79.552    66.50                                                 7              117.370   25.44 1.60548                                                                              60.5                                      8              -155.853  1.06                                                  9              -166.538  3.68  1.62409                                                                              36.0                            G.sub.2   10             -347.184  25.92                                                 11             172.816   29.58 1.51825                                                                              63.9                                      12             -521.651  29.44                                                 13             -59.855   5.32  1.58975                                                                              30                                        14    Liq      ∞   8.41                                        G.sub.3   15    FP       ∞   16.13 1.54212                                         16             ∞                                               ______________________________________                                         Aspherical coefficient                                                                               Exponent                                                 ______________________________________                                         13th surface                                                                   K = -3.31488 × 10.sup.- 1                                                A1 = 8.39622 × 10.sup.-8                                                                       P1 = 4.000                                               A2 = 5.93244 × 10.sup.-11                                                                      P2 = 6.000                                               A3 = 3.13441 × 10.sup.-15                                                                      P3 = 8.000                                               A4 = 3.91019 × 10.sup.-20                                                                      P4 = 10.000                                              ______________________________________                                    

EXAMPLE 5

f=136.04 Aperture ratio 1:1.2 Magnification -0.125268 Half field angle 21.64° f/f_(2max) =1.28

    ______________________________________                                                                  ri        di    ni    νi                           ______________________________________                                                   1              189.391   13.00 1.72391                                                                              43.7                                      2              -8102.491 2.20                                                  3              80.810    15.28 1.69661                                                                              53.2                            G.sub.1   4              180.905   7.01                                                  5              1321.456  4.38  1.73429                                                                              28.5                                      6              73.787    49.91                                                 7              95.211    27.00 1.60548                                                                              60.7                                      8              -176.262  7.86                                                  9              -170.556  3.50  1.58975                                                                              30                              G.sub.2   10             -615.824  35.97                                                 11             370.742   5.00  1.49410                                                                              57                                        12             -326.152  30.46                                                 13             -60.847   3.50  1.58975                                                                              30                                        14    Liq      -90.000   15.99 1.41000                               G.sub.3   15    FP       ∞   7.00  1.54212                                         16             ∞                                               ______________________________________                                         Aspherical coefficient                                                                               Exponent                                                 ______________________________________                                         9th surface                                                                    K = -3.16000 ×  10.sup.-1                                                A1 = -8.83784 × 10.sup.-9                                                                      P1 = 4.000                                               A2 = -2.40810 × 10.sup.-13                                                                     P2 = 6.000                                               A3 = -2.36390 × 10.sup.-18                                                                     P3 = 8.000                                               A4 = -1.09501 × 10.sup.-21                                                                     P4 = 10.000                                              11th surface                                                                   K = 0.0                                                                        A1 = -4.90017 × 10.sup.-7                                                                      P1 = 4.000                                               A2 = -1.00924 × 10.sup.-10                                                                     P2 = 6.000                                               A3 = -2.63563 × 10.sup.-15                                                                     P3 = 8.000                                               A4 = -8.40265 × 10.sup.-18                                                                     P4 = 10.000                                              12th surface                                                                   K = 0.0                                                                        A1 = -6.19209 × 10.sup.-8                                                                      P1 = 4.000                                               A2 = 1.21883 × 10.sup.-11                                                                      P2 = 6.000                                               A3 = -3.59893 × 10.sup.-15                                                                     P3 = 8.000                                               A4 = 8.93348 × 10.sup.-18                                                                      P4 = 10.000                                              13th surface                                                                   K = -6.63848 × 10.sup.-1                                                 A1 = 2.84077 × 10.sup.-7                                                                       P1 = 4.000                                               A2 = -5.70865 × 10.sup.-11                                                                     P2 = 6.000                                               A3 = 6.52601 × 10.sup.-14                                                                      P3 = 8.000                                               A4 = -1.38890 × 10.sup.-17                                                                     P4 = 10.000                                              ______________________________________                                    

In the projection lenses for a projector shown in Examples 6 to 8, as shown in FIGS. 8 to 10, the first lens unit comprises, in order from the screen side, the positive first lens, the positive second lens and the negative third lens; the second lens unit comprises the positive fourth lens, the positive fifth lens, the positive sixth lens and the seventh lens having a small refracting power with a concave surface directed at the screen side; and the third lens unit comprises the negative eighth lens with a concave surface directed at the screen side. At least the sixth lens or the seventh lens has one or more aspheridal surfaces.

These Examples has the basic structure as described above, and are desired that as the auxiliary conditions, the following conditions are fulfilled.

(5) ν₁ >45, ν₃ <40

0.3<f/f₁ <0.7

0.5<f/f₂ <0.9

-1.5<f/f₃ <-1.0

where

ν₁ : Abbe's number of the first lens

ν₂ : Abbe's number of the second lens

ν₃ : Abbe's number of the third lens

f: Focal length of the entire system

f₁ : Focal length of the first lens

f₂ : Focal length of the second lens

f₃ : Focal length of the third lens ##EQU2## f₄ : Focal length of the fourth lens f₅ : Focal length of the fifth lens

₄ : Abbe's number of the fourth lens

₅ : Abbe's number of the fifth lens

(7) 0.2 D / f 0.5

D: On-axial air space between the third and fourth lenses

f: Focal length of the entire system

The seventh lens having a small refracting power with a concave surface directed at the screen side and the negative eighth lens are used to obtain a better image surface. Particularly, the seventh lens, in the case where the eighth lens is a spherical lens, makes it possible to obtain a good image surface not only in the case where the CRT fluorescent surface has a curvature but in the case where the fluorescent surface is flat.

The aforesaid sixth lens and seventh lens have aspherical surfaces, the former being used to correct the meridional coma mainly at a wide angle, and the latter being used to well correct the image surface.

The condition (5) is provided to enhance the practical performance and is related to the Abbe's number of the first, second and third lenses, the focal length of the entire system and the focal length of the first, second and third lenses. By fulfilling this condition, the chromatic aberrations including magnification occurring in the first and second lenses are well corrected.

The condition (6) is the condition relating to the Abbe's number and the focal length of the fourth lens and the Abbe's number and the focal length of the fifth lens, which makes it possible to well correct on-axial chromatic aberration occurring in the fifth lens having a considerable portion of the refracting power of the entire system.

The condition (7) is the condition relating to on-axial air space between the third and fourth lenses and the focal length of the entire system. When the value exceeds the lower limit of this condition, a sagital flare markedly increases, and the contrast and resolving power at a wide angle greatly lowers. When the value exceeds the upper limit, it becomes difficult to make the system compact. In the case where the field angle is widened, it becomes difficult to obtain a quantity of the margined light, making it difficult to obtain an uniform quality of image.

These Examples are specifically shown below.

The sixth lens has aspherical surfaces having an inflection point, and the seventh lens has aspherical surfaces having no inflection point. The first, second, third, fourth, fifth and eighth lenses are formed from sperical lenses, respectively.

EXAMPLE 6

Focal length f=136.04 Half field angle 21°05'

Aperture ratio 1:1.2

Magnifying magnification 6.35 times

f/f₁ =0.51 f/f₂ =0.75

f/f₃ =-1.39

    ______________________________________                                          ##STR1##                                                                                          r         d     n      ν                                ______________________________________                                                     1       222.410   13.00                                            L1                                  1.69686                                                                               53.2                                            2       -1164.460 1.00                                                         3       75.059    15.30                                            L2                                  1.69686                                                                               53.2                                            4       170.733   8.00                                                         5       1943.904  4.50                                             L3                                  1.70490                                                                               30.1                                            6       66.589    47.55                                                        7       98.372    27.00                                            L4                                  1.59161                                                                               61.3                                            8       -142.271  8.25                                                         9       -145.689  3.50                                             L5                                  1.62443                                                                               36.3                                           10       -386.033  35.00                                                       11       -7268.357 7.00                                             L6                                  1.49425                                                                               55.0                                           12       -130.565  21.05                                                       13       - 67.941  3.50                                             L7                                  1.49425                                                                               55.0                                           14       -91.631   10.00                                                       15       -88.576   3.00                                             L8                                  1.62443                                                                               36.3                                           16 Liq         ∞                                                                              20.80 1.41000                                             17             ∞                                              FP                              7.00  1.54230                                             18             ∞                                              ______________________________________                                         Aspherical coefficient                                                                               Exponent                                                 ______________________________________                                         11th surface                                                                   K = 0.0                                                                        A1 = -7.81559 × 10.sup.-7                                                                      P1 = 4.0000                                              A2 = 7.02230 × 10.sup.-12                                                                      P2 = 6.0000                                              A3 = -4.57050 × 10.sup.-15                                                                     P3 = 8.0000                                              A4 = -1.52311 × 10.sup.-18                                                                     P4 = 10.0000                                             12th surface                                                                   K = 0.0                                                                        A1 = -2.62554 × 10.sup.-7                                                                      P1 = 4.0000                                              A2 = 1.07062 × 10.sup.-10                                                                      P2 = 6.0000                                              A3 = 3.82736 × 10.sup.-14                                                                      P3 = 8.0000                                              A4 = -2.92188 × 10.sup.-18                                                                     P4 = 10.0000                                             13th surface                                                                   K = -2.71719 × 10.sup.-1                                                 A1 = 3.24894 × 10.sup.-7                                                                       P1 = 4.0000                                              A2 = -2.84276 × 10.sup.-11                                                                     P2 = 6.0000                                              A3 = 7.41442 × 10.sup.-14                                                                      P3 = 8.0000                                              A4 = -2.90580 × 10.sup.-17                                                                     P4 = 10.0000                                             14th surface                                                                   K = 1.31856                                                                    ______________________________________                                    

EXAMPLE 7

Focal length f=136.03 Half field angle 20°17'

Aperture ratio 1:1.2

Magnifying magnification 6.9 times

f/f₁ =0.54 f/f₂ =0.60

f/f₃ =-1.24

    ______________________________________                                          ##STR2##                                                                                          r         d     n      ν                                ______________________________________                                                     1       212.021   14.50                                            L1                                  1.71641                                                                               53.9                                            2       -1140.618 1.00                                                         3       85.305    15.00                                            L2                                  1.69686                                                                               53.2                                            4       173.340   10.50                                                        5       ∞   4.00                                             L3                                  1.70490                                                                               30.1                                            6       77.595    56.00                                                        7       92.547    26.00                                            L4                                  1.59161                                                                               61.3                                            8       -121.600  3.50                                             L5                                  1.62443                                                                               36.3                                            9       -274.568  35.80                                                       10       1151.150  6.50                                             L6                                  1.49425                                                                               55.0                                           11       -233.153  23.73                                                       12       -95.121   7.00                                             L7                                  1.49425                                                                               55.0                                           13       -100.986  6.50                                                        14       -71.996   3.20                                             L8                                  1.62443                                                                               36.3                                           15 Liq         ∞                                                                              20.80 1.41000                                             16             ∞                                              FP                              7.00  1.54230                                             17             ∞                                              ______________________________________                                         Aspherical coefficient                                                                               Exponent                                                 ______________________________________                                         10th surface                                                                   K = 0.0                                                                        A1 = -7.35810 × 10.sup.-7                                                                      P1 = 4.0000                                              A2 = -3.65500 × 10.sup.-11                                                                     P2 = 6.0000                                              A3 = 9.84940 × 10.sup.-15                                                                      P3 = 8.0000                                              A4 = -1.61460 × 10.sup.-18                                                                     P4 = 10.0000                                             11th surface                                                                   K = 0.0                                                                        A1 = -2.33970 × 10.sup.-7                                                                      P1 = 4.0000                                              A2 = 4.30220 × 10.sup.-11                                                                      P2 = 6.0000                                              A3 = 5.17890 × 10.sup.-14                                                                      P3 = 8.0000                                              A4 = -3.65070 × 10.sup.-18                                                                     P4 = 10.0000                                             12th surface                                                                   K = 4.01540 × 10.sup.-1                                                  A1 = 2.71610 × 10.sup.-7                                                                       P1 = 4.0000                                              A2 = -4.94340 × 10.sup.-11                                                                     P2 = 6.0000                                              A3 = 7.71990 × 10.sup.-14                                                                      P3 = 8.0000                                              A4 = -2.70880 × 10.sup.-17                                                                     P4 = 10.0000                                             13th surface                                                                   K = 1.70500                                                                    A1 = -8.79020 × 10.sup.-8                                                                      P1 = 4.0000                                              A2 = -1.32130 × 10.sup.-11                                                                     P2 = 6.0000                                              A3 = 1.34030 × 10.sup.-15                                                                      P3 = 8.0000                                              A4 = 8.36760 × 10.sup.-20                                                                      P4 = 10.0000                                             ______________________________________                                    

EXAMPLE 8

Focal length f=136.01 Half field angle 20°54'

Aperture ratio 1:1.2

Magnifying magnification 6.35 times

f/f₁ =0.56 f/f₂ =0.64

f/f₃ =-1.25

    ______________________________________                                          ##STR3##                                                                                          r         d     n      ν                                ______________________________________                                                     1       192.317   14.00                                            L1                                  1.71641                                                                               53.9                                            2       -1893.531 1.00                                                         3       82.736    15.00                                            L2                                  1.69686                                                                               53.2                                            4       174.442   8.50                                                         5       2117.169  4.50                                             L3                                  1.70490                                                                               30.1                                            6       74.059    56.00                                                        7       96.682    23.00                                            L4                                  1.59161                                                                               61.3                                            8       -136.337  1.20                                                         9       -134.911  3.50                                             L5                                  1.62443                                                                               36.3                                           10       -284.008  42.26                                                       11       3693.241  7.00                                             L6                                  1.49425                                                                               55.0                                           12       -186.460  20.49                                                       13       -78.200   7.00                                             L7                                  1.49425                                                                               55.0                                           14       -85.410   5.00                                                        15       -71.409   3.00                                             L8                                  1.62443                                                                               36.3                                           16 Liq         ∞                                                                              20.80 1.41000                                             17 FP          ∞                                              FP                              7.00  1.54230                                             18             ∞                                              ______________________________________                                         Aspherical coefficient                                                                               Exponent                                                 ______________________________________                                         11th surface                                                                   K = 0.0                                                                        A1 = -8.06844 × 10.sup.-7                                                                      P1 = 4.0000                                              A2 = -3.87241 × 10.sup.-13                                                                     P2 = 6.0000                                              A3 = -3.18224 × 10.sup.-15                                                                     P3 = 8.0000                                              A4 = -1.51116 × 10.sup.-18                                                                     P4 = 10.0000                                             12th surface                                                                   K = 0.0                                                                        A1 = -2.96426 × 10.sup.-7                                                                      P1 = 4.0000                                              A2 = 1.03691 × 10.sup.-10                                                                      P2 = 6.0000                                              A3 = 3.70839 × 10.sup.-14                                                                      P3 = 8.0000                                              A4 = -2.93265 × 10.sup.-18                                                                     P4 =  10.0000                                            13th surface                                                                   K = -3.97428 × 10.sup.-1                                                 A1 = 3.29946 × 10.sup.-7                                                                       P1 = 4.0000                                              A2 = -4.53491 × 10.sup.-11                                                                     P2 = 6.0000                                              A3 = 7.43436 × 10.sup.-14                                                                      P3 = 8.0000                                              A4 = -2.90521 × 10.sup.-17                                                                     P4 = 10.0000                                             14th surface                                                                   K = 1.12002                                                                    ______________________________________                                    

The present invention provides a projection lens for a projector in which the image forming performance is excellent and which can obtain an image quality of high definition as in a high quality television with a relatively simple construction despite the fact that the aperture ratio is large, about 1.2, and the half field angle is wide, about 21° as shown in the Examples and various aberration diagrams. 

What is claimed is:
 1. A projection lens for a projector comprising, in order from the screen side, a positive first lens unit including at least one positive lens and at least one negative lens, a positive second lens unit including at least one positive lens and at least one negative lens, and a negative third lens unit with a concave surface having a large curvature directed at the screen side, characterized in that said first lens unit comprises spherical lens elements, the at least one negative lens in the first lens unit having a concave surface having a larger curvature than said large curvature of said third lens unit, said larger curvature being directed at an adjacent convex surface farther from the screen, and the second lens unit and the third lens unit together having at least one or more aspherical surfaces.
 2. The projection lens for a projector according to claim 1, wherein the positive lens having the largest refracting power in said second lens unit comprises a glass lens.
 3. The projection lens for a projector according to claim 1, wherein the second lens unit includes a plastic positive lens and a plastic negative lens.
 4. The projection lens for a projector according to claim 1, wherein the combination of lens units satisfies the conditions below: ν₁₋ <40, ν₂₋ <40 where ν₁₋ : Abbe's number of a negative lens in the first lens unitν₂₋ : Abbe's number of a negative lens in the second lens unit.
 5. The projection lens for a projector according to claim 1, wherein the combination of lens units satisfies the condition below:0.7<f/f_(2max) <1.5 wheref: Focal length of the entire system f_(2max) : Focal length of the positive lens having the largest refracting power in the second lens unit.
 6. The projection lens for a projector according to claim 1, wherein the combination of lens units satisfies the conditions below:ν₁ <45, ν₂ >45, ν₃ >40 0.3-f/f₁ <0.7 0.5<f/f₂ <0.9 -1.5<f/f₃ <-1.0 whereν_(i) : Abbe's number of the i-th lens f: Focal length of the entire system f_(i) : Focal length of the i-th lens.
 7. The projection lens for a projector according to claim 1, wherein said positive second lens unit is composed of a positive fourth lens, a seventh lens having a small refracting power with a concave surface directed at the screen side, the sixth lens and the seventh lens together having one or more aspherical surfaces.
 8. The projection lens for a projector according to claim 1, wherein the lens satisfies the condition below: ##EQU3## where f₄ : Focal length of the fourth lensf₅ : Focal length of the fifth lens ν.sub. : Abbe's number of the fourth lens ν₅ : Abbe's number of the fifth lens.
 9. The projection lens for a projector according to claim 1, wherein the combination of lens units satisfies the condition below:0.2<D/f-0.5 whereD: On-axial air space between the thirds and fourth lenses f: Focal length of the entire system. 